1. Field of the Invention
This invention relates to micromachined silicon sensors or Micro Electro Mechanical Systems (MEMS) gas sensing technology that measures the quality of gases. The present invention also relates to gas concentration sensors of such gases. This invention additionally provides the design and make of a micromachined gas concentration sensor. The present invention specifically relates design and process of making the same for an oxygen concentration sensor using semiconductor solid electrolyte for applications in medical oxygen delivery, automotive fuel efficiency and other industrial emission control equipment.
2. Description of the Related Art
Oxygen concentration sensors have been widely used in gaseous environment such as analytical instrumentation, medical, equipment, automotive exhaust electronic contral unit (ECU), industrial emission control and environment control. However, most of the current oxygen sensors are manufactured with ceramic solid electrolyte or electrochemical cells that suffer a long response time and limited life time. High accuracy oxygen concentration measurement can be achieved using paramagnetic oxygen sensing technology but it is extremely sensitive to vibration and bulk in size in addition to its high cost. For many of the medical applications, response time is critical but the current technologies are yet to offer a practical solution. Oxygen sensor is also a critical component for emission control in an automotive ECU system where the amount of the oxygen at exhaust is used for adjustment of the fuel supply. Such a sensor is made of high temperature ceramic substrate with the yttrium stabilized zirconium oxide as the solid electrolyte and platinum as the electrode in the formality of a Nerost cell. (R. Ramamoorthy, P. K. Dutta, and S. A. Akbar, Oxygen sensors: materials, methods, designs and applications, J. Mater. Sci., 38 (2003) page 4271). The oxygen sensors for ECU system has been mass deployed since mid-1970s and the ECU systems for all cars have been, employed with the oxygen sensors (Topp, B. et aL, Methods for producing oxygen-sensing element, particularly for use with internal combustion engine exhaust emission analysis. U.S. Pat. No. 3,978,006 Aug. 31, 1976; Gold, T. J. et at, Exhaust electrode process for exhaust gas oxygen sensor, U.S. Pat. No. 4,303,490, Dec. 1, 1981; Watson, 1, Exhaust gas oxygen sensor diagnostic method and apparatus, U.S. Pat. No. 8,290,688 Oct. 16, 2012; Mizutani, A. et al., Oxygen sensor, U.S. Pat. No. 6,182,498, Feb. 6, 2001). However, the ceramic based sensor is not only limited to application of low oxygen concentration measurement but also costly for other applications such as motorcycle emission control. The same approaches of the oxygen sensor made on ceramics may take an alternative design utilizing the properties of amperometric characteristics of the zirconia oxide. At high temperature (often over 600° C.), zirconia oxide becomes a conductor to the oxygen ions and current passing through the electrodes shall be proportional to the oxygen concentration. The high temperature requirements of the ceramic oxygen sensors is however a drawback for ambient temperature applications as the local high temperature at the sensor is not desired and power requirement is large. The high temperature requirements also limit the capability of sensor portability as the power shall not be sustainable with most of the available portable energy sources.
Cole et aL (Cole, B. E., Nguyen, Q, and Bonne U., Rugged O2 microsensor, U.S. Pat. No. 5,282,948, Feb. 1, 1994) teaches an oxygen sensor with a silicon substrate and diaphragm made on the substrate containing platinum, silicon nitride and zirconia oxide. This structure is however in tact would be fragile as the limitation for the maximum silicon nitride thickness would be less than 1500 nm and the platinum interlace with silicon and silicon nitride thin film could suffer instability at the elevated temperature that is required for the oxygen sensing reaction for the zirconia oxide. It further reported (Cole, B. E., Uk, E., Schuldt, S., and Bonne, U., Oxygen microsensor development, GRI Tech Report 86/0190, 1986) that such a structure would produce high compressive stress making the sensor unpractical for manufacture. In a later disclosure (Aagard, R. L., Bonne, U. and Cole, B. E., Solid-state oxygen microsensor and thin structure therefor, U.S. Pat. No. 5,389,225, Feb. 14, 1995) by Agards et aL it proposed a solid-state oxygen microsensor which measures the potential difference generated by zirconia oxide based solid electrolyte in a preferably constant temperature gradient. Nonetheless, it did not establish a conventional approach for the control of the temperature gradient and thus making the feasibility of practical manufacture of such is unclear. Further the above disclosed oxygen sensors are most suitable for combustion and emission control applications but not for an oxygen sensor with wide dynamic range and last response time that could be applied for general purpose oxygen concentration measurement.
The said researches and disclosures have yet to produce a general, purpose oxygen sensor except for the λ sensor used in the automotive ECU system mostly as an on/off switch. The other oxygen sensors using paramagnetic principle are too costly for the said applications and are best for laboratory use with extremely sensitive to vibration and environmental instabilities.